Imaging devices, including charge coupled devices (CCD) and complementary metal oxide semiconductor (CMOS) circuits, among others, have commonly been used in photo-imaging applications. An imager circuit includes a focal plane array of pixels, each one of the pixels including a photosensor, for example, a photogate, photoconductor or a photodiode for accumulating photo-generated charge in the specified portion of the substrate.
Pixels in an imaging device function to collect light from a portion of a scene or image being captured by the imaging device. Lenses may be used to enhance the collection of light at various levels of the image capture process by focusing incoming light onto specific light-collecting portions of the device, thereby decreasing the amount of light lost and increasing the fidelity of the captured image. Accordingly, a microlens may be positioned above each pixel in a pixel array to focus incoming light on the photo-sensitive portion of the pixel. At a higher hardware level, an imaging device may further include one or more larger lenses positioned above the entire pixel array for imaging a scene onto the pixel array.
Lenses used in wireless telephones (e.g., cellular telephones) present a unique challenge. They must be compact, inexpensive, used with a digital pixel array and provide a high-quality picture. To achieve these goals, designers have used wafer-level optics (WLO), which involves packaging small lenses with the digital circuitry, including a pixel array. Currently, however, lenses used in WLO are highly aspheric, or “free form,” often with aspheric coefficients of the 10th to 14th order. Lenses may be created at a wafer level using a “lens master wafer.” A lens master wafer includes a plurality of lens-shaped dies. These lens shapes may be replicated in lens material on wafers to form lens wafers, which may then be used to form individual lens units for use with respective imaging devices.
One common way of forming a lens master wafer is to photodefine isolated sections in a polymer and reflow the sections to form lens shapes. The shape of the reflowed lens shapes is limited generally to ellipsoidal shapes using this method. In many cases, a different lens shape may be desired but cannot be achieved using this technique.
In an alternative method, lens shapes are formed using one or more single lens master stamps by, for example, precision diamond turning, and replicating the stamp shape through a step and repeat “stamp-and-step” method to populate an entire surface of the lens master wafer. Problems arise in the quality of the formed lens shapes, as the lens shapes suffer from variability and trapped air bubbles. Prior to stamping, the upper surface of the lens material layer is usually flat, which often contributes to the deformities and the inclusion of trapped air during the stamping process, resulting in imperfect formation of the desired lens shape. There is a need to minimize the variability in lens shapes across an area of a lens master wafer to be populated with lens with the required aspheric prescription shape.